Mask and forming method of mask

Abstract

The invention discloses a mask plate and a forming method thereof, and the mask plate comprises a substrate which comprises a first region and a second region; a reflective layer on the first region; and the absorbing layer is positioned on the second region, and the density of the material of the absorbing layer is smaller than that of the material of the reflecting layer. The mask plate is used in an X-ray reflection type exposure technology, so that the resolution ratio of photoetching is improved.

Description

Technical Field

[0001] The present invention relates to the field of semiconductor manufacturing, and particularly to a mask and a method for forming a mask. Background Art

[0002] A mask is an essential component in the lithography process. The mask carries a designed pattern, and light passes through the mask to transfer the designed pattern onto the photoresist. The performance of the mask directly determines the quality of the lithography process.

[0003] According to the different types of lithography machines used, masks are divided into transmissive masks used in projection lithography machines and reflective masks used in reflective lithography machines.

[0004] At present, the masks used in reflective lithography machines still need to be improved. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a mask and a method for forming a mask to improve the masks used in reflective lithography machines.

[0006] To solve the above technical problem, the technical solution of the present invention provides a mask, including: a substrate, the substrate including a first region and a second region; a reflective layer located on the first region; an absorption layer located on the second region, the density of the material of the absorption layer being less than the density of the material of the reflective layer.

[0007] Optionally, the thickness of the reflective layer is the same as the thickness of the absorption layer; the surface of the reflective layer is flush with the surface of the absorption layer.

[0008] Optionally, the thickness range of the reflective layer and the absorption layer is from 1 nanometer to 50 nanometers.

[0009] Optionally, it further includes: a buffer layer located on the substrate, the reflective layer and the absorption layer being located on the buffer layer, the transmittance of the material of the buffer layer being less than 2%.

[0010] Optionally, the material of the buffer layer includes a metal, the metal including one or a combination of more of tungsten, hafnium, tantalum, platinum, and gold.

[0011] Optionally, the density of the material of the reflective layer is greater than 15 grams per cubic centimeter.

[0012] Optionally, the material of the reflective layer includes a metal, the metal including one or a combination of more of tungsten, hafnium, tantalum, platinum, and gold.

[0013] Optionally, the density of the material of the absorption layer is less than 8 grams per cubic centimeter.

[0014] Optionally, the material of the absorption layer includes: silicon, silicon oxide, silicon carbide, or silicon germanium.

[0015] Optionally, it further includes: a protective layer located on the surface of the reflection layer and the surface of the absorption layer, the density of the protective layer is less than the density of the absorption layer, and the density of the protective layer is less than the density of the reflection layer.

[0016] Optionally, the material of the protective layer includes aluminum oxide, silicon nitride, or silicon oxide.

[0017] Optionally, the thickness of the protective layer is less than the thickness of the absorption layer, and the thickness of the protective layer is less than the thickness of the reflection layer.

[0018] Optionally, the thickness range of the protective layer is greater than 0 and less than or equal to 2 nanometers.

[0019] Optionally, the thermal expansion coefficient range of the material of the substrate is -1e-11 / K to 1e-11 / K.

[0020] Optionally, the material of the substrate includes silicon, silicon oxide, glass, or ceramic.

[0021] Correspondingly, the technical solution of the present invention further provides a method for forming a mask, including: providing a substrate, the substrate includes a first region and a second region; forming a reflection layer on the first region; forming an absorption layer on the second region, the density of the material of the absorption layer is less than the density of the material of the reflection layer.

[0022] Optionally, the thickness of the reflection layer is the same as the thickness of the absorption layer; the surface of the reflection layer is flush with the surface of the absorption layer.

[0023] Optionally, the method for forming the reflection layer includes: forming a reflection material layer on the substrate; forming a patterned mask layer on the reflection material layer, the patterned mask layer exposes the surface of the reflection material layer on the second region; etching the reflection material layer with the patterned mask layer as a mask, removing the reflection material layer on the second region, forming the reflection layer on the first region, and forming a groove between the reflection layers.

[0024] Optionally, the method for forming the absorption layer includes: forming an absorption material layer in the groove and on the surface of the reflection layer; planarizing the absorption material layer until the surface of the reflection layer is exposed, and forming the absorption layer in the groove, the absorption layer is located on the second region.

[0025] Optionally, the process for forming the reflection material layer includes physical vapor deposition process, chemical vapor deposition process, or atomic layer deposition process.

[0026] Optionally, the process of forming the absorption material layer includes chemical vapor deposition process, atomic layer deposition process or high aspect ratio process.

[0027] Optionally, before forming the reflective layer on the first region, it further includes: forming a buffer layer on the substrate, the reflective layer and the absorption layer are located on the buffer layer, and the transmittance of the buffer layer material is less than 2%.

[0028] Optionally, it further includes: forming a protective layer on the surface of the reflective layer and the surface of the absorption layer, the density of the protective layer is less than the density of the absorption layer, and the density of the protective layer is less than the density of the reflective layer.

[0029] Optionally, the thickness of the protective layer is less than the thickness of the absorption layer, and the thickness of the protective layer is less than the thickness of the reflective layer.

[0030] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0031] For the mask of the present invention, the density of the material of the absorption layer is less than the density of the material of the reflective layer. The density of the material of the reflective layer is relatively large, so that the critical angle of X-rays on the surface of the reflective layer is large, and the density of the material of the absorption layer is relatively small, and the critical angle of X-rays on the surface of the absorption layer is small. By setting the incident angle of X-rays incident on the surface of the mask, making the incident angle greater than the second critical angle and less than the first critical angle, when X-rays are reflected from the surface of the mask to the surface of the photoresist, different reflectivities can be obtained on the surface of the reflective layer and the surface of the absorption layer. Specifically, the reflectivity of X-rays on the surface of the reflective layer is greater than the reflectivity on the surface of the absorption layer, so that the difference in the light intensity of X-rays reflected from different regions of the mask to the surface of the photoresist is greater, and the difference in the reaction degree of the photoresist is greater, so as to realize the transfer of the pattern of the mask to the photoresist and improve the resolution of lithography.

[0032] Furthermore, the mask further includes a buffer layer located on the substrate, and the reflective layer and the absorption layer are located on the buffer layer. The buffer layer can absorb X-rays passing through the reflective layer and the absorption layer and is not prone to generating secondary electrons.

[0033] Furthermore, the mask further includes a protective layer located on the surface of the reflective layer and the surface of the absorption layer, the density of the protective layer is less than the density of the absorption layer, and the density of the protective layer is less than the density of the reflective layer. The protective layer is used to protect the surfaces of the reflective layer and the absorption layer. The density of the protective layer is relatively small, so that X-rays are not likely to be reflected at the interface of the protective layer and will only be reflected at the interfaces of the reflective layer and the absorption layer. Description of the Drawings

[0034] Figure 1 is a schematic diagram of a mask in an embodiment;

[0035] Figures 2 to 6 is a schematic diagram of the formation process of a mask in an embodiment of the present invention;

[0036] Figure 7 is a schematic diagram of the formation process of a mask in another embodiment of the present invention. Detailed implementation manners

[0037] As described in the background art, the mask used in a reflective lithography machine still needs to be improved.

[0038] Specifically, the resolution of lithography where k1 is a process-related factor, k1 is a constant, λ is the optical wavelength of the light source, and NA is the numerical aperture. It can be known that the resolution CD of lithography can be increased by increasing the numerical aperture NA.

[0039] However, due to the process window k2 is a process-related factor, k2 is a constant, the larger the numerical aperture NA, the smaller the process window D F will also decrease accordingly, and the process window D F measures the application range of the lithography technology. When the process window D F decreases, the application of this lithography technology is limited.

[0040] Therefore, the resolution CD of lithography can be improved by reducing the light wavelength λ. The smaller the light wavelength λ, the smaller the resolution CD of lithography. An X-ray with a smaller wavelength range is used as the exposure light source.

[0041] At present, the commonly used light source is extreme ultraviolet light (EUV). An X-ray with a smaller wavelength range is used as the exposure light source, and the X-ray wavelength range is from 1 picometer to 10 nanometers. However, the commonly used lithography machine is a projection lithography machine. Please refer to Figure 1 , the projection mask for the projection lithography machine includes a substrate 300. The substrate 300 includes a light-transmitting area and a light-shielding area. A light-shielding layer 301 is provided on the light-shielding area. The light-shielding layer 301 is used to block light, and the light irradiates the photoresist layer from the light-transmitting area.

[0042] However, due to the strong penetrability of X-rays, the light-shielding layer 301 cannot block the penetration of X-rays. The light transmitted through the light-transmitting area by the X-ray is T1, and the light transmitted through the light-shielding area is T2. Both T1 and T2 can cause the photoresist to react to form a pattern, and thus the patterns of the light-transmitting area and the light-shielding area cannot be reflected on the photoresist layer.

[0043] To solve the above problems, the technical solution of the present invention provides a mask and a method for forming a mask, wherein the density of the material of the absorption layer is less than the density of the material of the reflection layer. The material of the reflection layer has a relatively large density, so that the critical angle of X-rays on the surface of the reflection layer is large, and the material of the absorption layer has a relatively small density, and the critical angle of X-rays on the surface of the absorption layer is small. By setting the incident angle of X-rays incident on the surface of the mask, making the incident angle greater than the second critical angle and less than the first critical angle, when X-rays are reflected from the surface of the mask to the surface of the photoresist, different reflectivities can be obtained on the surface of the reflection layer and the surface of the absorption layer. Specifically, the reflectivity of X-rays on the surface of the reflection layer is greater than the reflectivity on the surface of the absorption layer, so that the difference in the light intensity of X-rays reflected from different regions of the mask to the surface of the photoresist is greater, and the difference in the reaction degree of the photoresist is greater, so as to realize the transfer of the pattern of the mask to the photoresist and improve the resolution of lithography.

[0044] To make the above objects, features, and beneficial effects of the present invention more obvious and understandable, the following detailed description will be given to the specific embodiments of the present invention with reference to the accompanying drawings.

[0045] Figures 2 to 6 It is a schematic diagram of the formation process of the mask in an embodiment of the present invention.

[0046] Please refer to Figure 2 , a substrate 100 is provided, and the substrate 100 includes a first region I and a second region II.

[0047] The thermal expansion coefficient range of the material of the substrate 100 is from -1e-11 / K to 1e-11 / K.

[0048] The thermal expansion coefficient of the substrate 100 is relatively small, so that the mask is relatively stable during use, and the pattern formed by the reflection layer and the absorption layer on the mask will not be affected by the deformation of the substrate 100.

[0049] The material of the substrate 100 includes silicon, silicon oxide, glass, or ceramic.

[0050] In this embodiment, the material of the substrate 100 includes silicon.

[0051] Please continue to refer to Figure 2 , a buffer layer 101 is formed on the substrate 100; a reflective material layer 102 is formed on the buffer layer 101.

[0052] The buffer layer 101 can absorb the X-rays that subsequently pass through the reflection layer and the absorption layer and is not likely to generate secondary electrons.

[0053] In this embodiment, the transmittance of the material of the buffer layer 101 is less than 2% so that the buffer layer 101 can absorb X-rays.

[0054] The material of the buffer layer 101 includes a metal, and the metal includes one or a combination of more than one of tungsten, hafnium, tantalum, platinum, and gold.

[0055] In this embodiment, the material of the buffer layer 101 includes tungsten.

[0056] In this embodiment, the thickness of the buffer layer 101 is greater than the thickness of the subsequent formed reflective layer, and the thickness of the buffer layer 101 is greater than the thickness of the subsequent formed absorption layer, so that the buffer layer 101 absorbs the X-rays that subsequently pass through the reflective layer and the absorption layer. In other embodiments, the buffer layer may not be formed.

[0057] The reflective material layer 102 is used to form a reflective layer on the first region I subsequently. The material of the reflective material layer 102 is different from the material of the buffer layer 101, and the thickness of the reflective material layer 102 is less than the thickness of the buffer layer 101.

[0058] In this embodiment, the density of the material of the reflective material layer 102 is greater than 15 grams per cubic centimeter.

[0059] The material of the reflective material layer 102 includes a metal, and the metal includes: one or a combination of more than one of tungsten, hafnium, tantalum, platinum, and gold.

[0060] In this embodiment, the material of the reflective material layer 102 includes tantalum.

[0061] The process of forming the reflective material layer 102 includes physical vapor deposition process, chemical vapor deposition process, or atomic layer deposition process.

[0062] In this embodiment, the process of forming the reflective material layer 102 includes physical vapor deposition process.

[0063] Please refer to Figure 3 , etch the reflective material layer 102 until the surface of the buffer layer 101 is exposed, and form a reflective layer 103 on the first region I, and the reflective layer 103 is located on the buffer layer 101.

[0064] The forming method of the reflective layer 103 includes: forming a patterned mask layer (not shown) on the reflective material layer 102, and the patterned mask layer exposes the surface of the reflective material layer 102 on the second region II; etching the reflective material layer 102 with the patterned mask layer as a mask, removing the reflective material layer 102 on the second region II, forming the reflective layer 103 on the first region I, and forming grooves 104 between the reflective layers 103.

[0065] The process of etching the reflective material layer 102 using the patterned mask layer as a mask includes a dry etching process, and the dry etching process can obtain a reflective layer 103 with a better sidewall topography and a higher dimensional accuracy.

[0066] In this embodiment, the groove 104 is located on the second region II.

[0067] Please refer to Figure 4 , an absorption material layer 105 is formed in the groove 104 and on the surface of the reflective layer 103.

[0068] The density of the material of the absorption material layer 105 is less than the density of the material of the reflective layer 103.

[0069] In this embodiment, the density of the material of the absorption material layer 105 is less than 8 grams per cubic centimeter.

[0070] The material of the absorption material layer 105 includes: silicon, silicon oxide, silicon carbide, or silicon germanium.

[0071] In this embodiment, the material of the absorption material layer 105 includes silicon oxide.

[0072] The absorption material layer 105 is used to form an absorption layer on the second region II subsequently.

[0073] The process of forming the absorption material layer includes a chemical vapor deposition process (Chemical Vapor Deposition, abbreviated as CVD), an atomic layer deposition process (Atomic Layer Deposition, abbreviated as ALD), or a high aspect ratio process (high aspect ratio process, abbreviated as HARP).

[0074] In this embodiment, the process of forming the absorption material layer includes a chemical vapor deposition process.

[0075] Please refer to Figure 5 and Figure 6 , Figure 6 is Figure 5 a top view of Figure 5 is Figure 6 a schematic structural diagram in the direction of the section line AA1 in

[0076] The absorption material layer 105 is planarized until the surface of the reflective layer 103 is exposed, and the absorption layer 106 is formed in the groove 104. The absorption layer 106 is located on the second region II, and the density of the material of the absorption layer 106 is less than the density of the material of the reflective layer 103.

[0077] In this embodiment, the thickness of the reflection layer 103 is the same as the thickness of the absorption layer 106; the surface of the reflection layer 103 is flush with the surface of the absorption layer 106.

[0078] The density of the material of the absorption layer 106 is less than the density of the material of the reflection layer 103. The density of the material of the reflection layer 103 is relatively large. The greater the material density, the higher the corresponding electron density. X-rays are scattered by electrons. On the surface of the reflection layer 103 with a high electron density, the X-rays are scattered too much and it is more difficult for them to penetrate the surface of the reflection layer 103 and enter the interior of the reflection layer 103. Therefore, the X-rays are more likely to form total reflection on the surface of the reflection layer 103, and thus the critical angle of the X-rays on the surface of the reflection layer 103 is large; conversely, the density of the material of the absorption layer 106 is small, and the critical angle of the X-rays on the surface of the absorption layer 106 is small.

[0079] Please continue to refer to Figure 5 In this embodiment, the X-rays have a first critical angle α1 on the surface of the first region. The first critical angle α1 is the angle between the X-rays incident from the air onto the surface of the reflection layer 103 and the surface of the reflection layer 103. When the incident angle of the X-rays incident from the air onto the surface of the reflection layer 103 is less than the first critical angle α1, the X-rays undergo total reflection on the surface of the reflection layer 103, and the reflectivity of the X-rays on the surface of the reflection layer 103 is high; the X-rays have a second critical angle α2 on the surface of the second region. The second critical angle α2 is the angle between the X-rays incident from the air onto the surface of the absorption layer 106 and the surface of the absorption layer 106. When the incident angle of the X-rays incident from the air onto the surface of the absorption layer 106 is less than the second critical angle α2, the X-rays undergo total reflection on the surface of the absorption layer 106.

[0080] In this embodiment, the first critical angle α1 is greater than the second critical angle α2.

[0081] In this embodiment, the incident angle range of the X-rays incident on the surface of the mask is 0 to 1.5 degrees. The incident angle is the angle between the X-rays and the surface of the mask. The incident angles of the X-rays incident on the surfaces of the reflection layer 303 and the absorption layer 306 are the same.

[0082] By adjusting the incident angle of the X-rays incident on the surface of the mask, such that the incident angle is greater than the second critical angle α2 and less than the first critical angle α1, in this way, the X-rays undergo total reflection on the surface of the reflection layer 103, and the reflectivity of the X-rays on the surface of the reflection layer 103 is high; the X-rays do not undergo total reflection on the surface of the absorption layer 106, and the reflectivity of the X-rays on the surface of the absorption layer 106 is low, so that the reflectivity of the X-rays on the surface of the first region is greater than the reflectivity on the surface of the second region, in order to obtain different reflectivities of the X-rays on the surfaces of the first region and the second region.

[0083] In this embodiment, the range of the first critical angle α1 is 0 to 1.5 degrees, and the range of the second critical angle α2 is 0 to 1.5 degrees.

[0084] By setting the incident angle of the X-ray, the first critical angle α1 is made greater than the second critical angle α2, and the incident angle of the X-ray is made to be between the second critical angle α2 and the first critical angle α1, so that the X-ray can obtain different reflectivities on the surfaces of the first region and the second region.

[0085] Thus, when the X-ray is reflected from the surface of the mask to the surface of the photoresist, different reflectivities can be obtained on the surface of the reflective layer 103 and the surface of the absorption layer 106. Specifically, the reflectivity of the X-ray on the surface of the reflective layer 103 is greater than that on the surface of the absorption layer 106, so that the difference in the light intensity of the X-ray reflected from different regions of the mask to the surface of the photoresist is greater, and the difference in the reaction degree of the photoresist is greater, so as to realize the transfer of the pattern of the mask to the photoresist and improve the resolution of lithography.

[0086] Please continue to refer to Figure 5 , the mask is used to reflect the incident X-ray on the surface of the mask to the surface of the photoresist layer. The X-ray reflected from the surface of the reflective layer 103 is the first reflected ray L1, and the X-ray reflected from the surface of the absorption layer 106 is the second reflected ray L2. The reflectivity of the X-ray on the surface of the reflective layer 103 is greater than that on the surface of the absorption layer 106.

[0087] Correspondingly, an embodiment of the present invention further provides a mask. Please continue to refer to Figure 5 and Figure 6 , including:

[0088] A substrate 100, the substrate 100 includes a first region I and a second region II;

[0089] A reflective layer 103 located on the first region I;

[0090] An absorption layer 106 located on the second region II, and the density of the material of the absorption layer 106 is less than the density of the material of the reflective layer 103.

[0091] In this embodiment, the thickness of the reflective layer 103 is the same as the thickness of the absorption layer 106; the surface of the reflective layer 103 is flush with the surface of the absorption layer 106.

[0092] In this embodiment, the thickness range of the reflective layer 103 and the absorption layer 106 is 1 nanometer to 50 nanometers.

[0093] In this embodiment, it further includes: a buffer layer 101 located on the substrate 100, the reflective layer 103 and the absorption layer 106 are located on the buffer layer 101, and the transmittance of the material of the buffer layer 101 is less than 2%.

[0094] In this embodiment, the material of the buffer layer 101 includes a metal, and the metal includes one or a combination of more than one of tungsten, hafnium, tantalum, platinum, and gold.

[0095] In this embodiment, the density of the material of the reflective layer 103 is greater than 15 g / cm³.

[0096] In this embodiment, the material of the reflective layer 103 includes a metal, and the metal includes one or a combination of more than one of tungsten, hafnium, tantalum, platinum, and gold.

[0097] In this embodiment, the density of the material of the absorption layer 106 is less than 8 g / cm³.

[0098] In this embodiment, the material of the absorption layer 106 includes: silicon, silicon oxide, silicon carbide, or silicon germanium.

[0099] In this embodiment, the thermal expansion coefficient range of the material of the substrate 100 is -1e-11 / K to 1e-11 / K.

[0100] Figure 7 It is a schematic diagram of the formation process of a mask in another embodiment of the present invention.

[0101] Please refer to Figure 7 , Figure 7 For the schematic diagram based on Figure 5 On the basis, a protective layer 207 is formed on the surface of the reflective layer 103 and the surface of the absorption layer 106. The density of the protective layer 207 is less than the density of the absorption layer 106, and the density of the protective layer 207 is less than the density of the reflective layer 103.

[0102] The protective layer 207 is used to protect the surfaces of the reflective layer 103 and the absorption layer 106. The density of the protective layer 207 is small, so that X-rays are not easily reflected on the surface of the protective layer 207 and will only be reflected on the surfaces of the reflective layer 103 and the absorption layer 106.

[0103] The thickness of the protective layer 207 is less than the thickness of the absorption layer 106, and the thickness of the protective layer 207 is less than the thickness of the reflective layer 103. So that it is easy for X-rays to pass through and be incident on the surface of the reflective layer 103 and the surface of the absorption layer 106.

[0104] In this embodiment, the thickness range of the protective layer 207 is greater than 0 and less than or equal to 2 nanometers.

[0105] In this embodiment, the material of the protective layer 207 includes aluminum oxide, silicon nitride, or silicon oxide.

[0106] Correspondingly, an embodiment of the present invention further provides a photomask. Please continue to refer to Figure 7 , Figure 7 The difference between the structure in Figure 5 and the structure in

[0107] is that the protective layer 207 is located on the surface of the reflective layer 103 and the surface of the absorption layer 106, and the density of the protective layer 207 is less than the density of the absorption layer 106, and the density of the protective layer 207 is less than the density of the reflective layer 103.

[0108] In this embodiment, the thickness of the protective layer 207 is less than the thickness of the absorption layer 106, and the thickness of the protective layer 207 is less than the thickness of the reflective layer 103.

[0109] In this embodiment, the thickness range of the protective layer 207 is greater than 0 and less than or equal to 2 nanometers.

[0110] Although the present invention is disclosed as above, the present invention is not limited thereto. Any person skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention should be determined by the scope defined by the claims.

Claims

1. A photomask, characterized in that, Comprising: A substrate, the substrate comprising a first region and a second region; A reflective layer located on the first region; An absorption layer located on the second region, the density of the material of the absorption layer being less than the density of the material of the reflective layer.

2. The reticle according to claim 1, wherein The thickness of the reflective layer is the same as the thickness of the absorption layer; the surface of the reflective layer is flush with the surface of the absorption layer.

3. The reticle according to claim 2, wherein The thickness of the reflective layer and the thickness of the absorption layer range from 1 nanometer to 50 nanometers.

4. The photomask according to claim 1, wherein Further comprising: A buffer layer located on the substrate, the reflective layer and the absorption layer being located on the buffer layer, the transmittance of the material of the buffer layer being less than 2%.

5. The mask according to claim 4, characterized in that The material of the buffer layer comprises a metal, the metal comprising one or a combination of more of tungsten, hafnium, tantalum, platinum and gold.

6. The mask according to claim 1, characterized in that, The density of the material of the reflective layer is greater than 15 grams per cubic centimeter.

7. The reticle according to claim 6, characterized in that The material of the reflective layer comprises a metal, the metal comprising one or a combination of more of tungsten, hafnium, tantalum, platinum and gold.

8. The reticle according to claim 1, wherein The density of the material of the absorption layer is less than 8 grams per cubic centimeter.

9. The reticle according to claim 8, wherein, The material of the absorption layer comprises silicon, silicon oxide, silicon carbide or silicon germanium.

10. The reticle according to claim 1, characterized in that, Further comprising: A protective layer located on the surface of the reflective layer and the surface of the absorption layer, the density of the protective layer being less than the density of the absorption layer and the density of the protective layer being less than the density of the reflective layer.

11. The reticle according to claim 10, wherein, The material of the protective layer comprises aluminum oxide, silicon nitride or silicon oxide.

12. The reticle according to claim 10, wherein The thickness of the protective layer is less than the thickness of the absorption layer and the thickness of the protective layer is less than the thickness of the reflective layer.

13. The reticle according to claim 12, wherein The thickness range of the protective layer is greater than 0 and less than or equal to 2 nanometers.

14. The reticle according to claim 1, wherein, The thermal expansion coefficient range of the material of the substrate is from -1e-11 / K to 1e-11 / K.

15. The photomask according to claim 14, characterized in that, The material of the substrate comprises silicon, silicon oxide, glass or ceramic.

16. A method for forming a mask, characterized in that, Comprising: Providing a substrate, the substrate comprising a first region and a second region; Forming a reflective layer on the first region; Forming an absorption layer on the second region, the density of the material of the absorption layer being less than the density of the material of the reflective layer.

17. The method for forming a photomask according to claim 16, wherein, The thickness of the reflective layer is the same as the thickness of the absorption layer; the surface of the reflective layer is flush with the surface of the absorption layer.

18. The method for forming a mask according to claim 16, characterized in that, The method for forming the reflective layer comprises: forming a reflective material layer on the substrate; forming a patterned mask layer on the reflective material layer, the patterned mask layer exposing the surface of the reflective material layer on the second region; etching the reflective material layer using the patterned mask layer as a mask, removing the reflective material layer on the second region, forming the reflective layer on the first region, and forming a groove between the reflective layers.

19. The method for forming a mask according to claim 18, characterized in that, The method for forming the absorption layer comprises: forming an absorption material layer in the groove and on the surface of the reflective layer; planarizing the absorption material layer until the surface of the reflective layer is exposed, and forming the absorption layer in the groove, the absorption layer being located on the second region.

20. The method for forming a mask according to claim 18, wherein The process for forming the reflective material layer comprises a physical vapor deposition process, a chemical vapor deposition process or an atomic layer deposition process.

21. The method for forming a mask according to claim 19, wherein The process for forming the absorption material layer comprises a chemical vapor deposition process, an atomic layer deposition process or a high aspect ratio process.

22. The method for forming a mask according to claim 16, wherein Before forming the reflective layer on the first region, further comprising: forming a buffer layer on the substrate, the reflective layer and the absorption layer being located on the buffer layer, the transmittance of the material of the buffer layer being less than 2%.

23. The method for forming a mask according to claim 16, wherein, Further comprising: A protective layer is formed on the surface of the reflective layer and the surface of the absorption layer. The density of the protective layer is less than the density of the absorption layer, and the density of the protective layer is less than the density of the reflective layer.

24. The method for forming a mask according to claim 23, wherein, The thickness of the protective layer is less than the thickness of the absorption layer, and the thickness of the protective layer is less than the thickness of the reflective layer.

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